Concentricity
Concentricity in GD&T controls how closely the median points of a cylindrical or rotational feature align with a datum axis or common centerline, not just surface appearance. It is used in CNC machining for bores, shafts, and sleeves to ensure minimal imbalance, uniform wall thickness, and proper assembly fit during rotation.
On the CNC lathe or mill, concentricity becomes critical when secondary features like bores, threads, or diameters must share the same true centerline as a primary datum. This is especially important for rotating parts such as pump components, motor shafts, and precision sleeves, where off-center material causes vibration, bearing wear, and assembly mismatch. Inspection requires a CMM to evaluate derived median points, not a simple dial indicator, because the tolerance zone is cylindrical and centered on the datum axis. Tight concentricity limits (e.g., 0.01–0.02 mm) reduce wobble, uneven loading, and friction in high-speed or precision assemblies. In woodworking or millwork, the same principle applies when routing or boring must keep holes centered to prevent misalignment, even if the drawing uses positional tolerances instead of formal GD&T.
Is concentricity a surface control or an axis control?
It is an axis/median-point control; the tolerance zone is cylindrical and centered on the datum axis, but the actual requirement is that the feature’s derived median points remain within that zone.
Why is concentricity harder to inspect than runout?
Because it requires determining derived median points across the feature, which usually needs CMM-type evaluation rather than a simple dial-indicator sweep.
What is the main functional reason to call it out on a drawing?
To control mass distribution and centerline alignment so the part rotates smoothly, assembles correctly, and avoids vibration or localized loading in service.